1. Field of Invention
The invention relates to a method and a device for determining at least operating parameter of a device for extracorporeal blood treatment as a function of the absolute pressure. For the sake of simplicity, the term blood treatment device is used hereinafter for all devices for extracorporeal treatment of blood.
2. Description of the Prior Art
Various types of blood treatment devices are known. The known blood treatment devices include, for example, devices for hemodialysis, hemofiltration and hemodiaflltration. During extracorporeal blood treatment, the blood flows through a blood treatment unit in an extracorporeal blood circulation. Of the devices for hemodialysis, hemofiltration and hemodiafiltration, the blood treatment unit is a dialyzer or filter which is separated by a semipermeable membrane into a blood chamber and a dialysis fluid chamber, when considered schematically. During the blood treatment by means of hemodialysis or hemodiafiltration, blood flows through the blood chamber while a dialysis fluid flows through the dialysis fluid chamber.
Fresh dialysis fluid may be supplied through a dialysis fluid system which is integrated into the device for extracorporeal blood treatment. Clean water, e.g., from reverse osmosis may be supplied to the dialysis fluid system after first being degassed and then mixed with liquid concentrates to prepare fresh dialysis fluid. Mixing may be accomplished, for example, by adding liquid concentrates to the clean water line at separate addition points and then mixing them thoroughly in a mixing chamber or by adding the liquid concentrates through separate feed points directly to a mixing chamber. The fresh dialysis fluid flows first through a balancing system and is then directed through the dialysis chamber of the dialyzer. The fresh dialysis fluid is then loaded with water and ingredients from the blood and thereby becomes spent dialysis fluid. After leaving the dialyzer, the spent dialysis fluid passes through the balancing system, where any difference between the volume of the fresh dialysis fluid and the spent dialysis fluid is determined.
The mixing chamber has incoming fluid lines and outgoing fluid lines. The mixing chamber may receive incompletely premixed mixture from degassed clean water and fluid concentrates. The complete mixing takes place in the mixing chamber. Fresh dialysis fluid is removed from the mixing chamber through a dialysis fluid line. Clean water, liquid concentrates and dialysis fluid are delivered in the lines by pumps. The clean water is degassed by creating a vacuum by means of a degassing pump in the clean water line upstream from the mixing chamber. The liquid concentrates are delivered by metering pumps upstream from the mixing chamber. The dialysis fluid is delivered through a dialysis fluid pump in the dialysis fluid line. With the dialysis fluid line upstream from dialyzer and with the dialysis fluid line downstream from the dialyzer, additional pumps may be in fluid connection, such as, for example, a flow pump in the dialysis fluid line and an ultrafiltration pump.
With the generic devices for extracorporeal blood treatment, inexpensive relative pressure sensors are usually used to measure the pressures. The pressures inside the dialysis fluid system are therefore usually set at ambient pressure. The generic devices for extracorporeal blood treatment therefore usually do not have an integrated absolute pressure gauge.
The operating parameters may be set as a function of the absolute pressure on known devices for extracorporeal blood treatment, which may be done either by a service technician using an external absolute pressure gauge or by storing approximate values in the central control unit.
The operating parameters are set by a service technician, who may carry an absolute pressure gauge with him for this purpose, as a function of the absolute pressure in setting up a blood treatment device in a selected geographic region, for example.
For example, according to the state of the art in setting up a blood treatment device, the elevation of the setup site is set through the choice of the value ranges and average values for the operating parameters which depend on the absolute pressure are preselected for each value range. For example, average values of the operating parameters depending on the absolute pressure are preselected for the following value ranges as a function of the elevation of the setup site above sea level:                setup height less than 800 meters above sea level (N.N.=normal zero of sea level),        setup height between 800 meters above sea level and 1400 meters above sea level,        setup height 1400 meters above sea level to 2000 meters above sea level,        setup height 2000 meters above sea level.        
The elevation of the setup site can usually be estimated roughly without any additional assistance. The operating parameters predetermined for the value range are set according to the selection of a value range.
The known methods for setting operating parameters depending on the absolute pressure on blood treatment devices are associated with disadvantages because they are either complex and difficult to automate or they are inaccurate and also depend on the reliability of a user input that is susceptible to errors.
Examples of such operating parameters, which depend on the absolute pressure include the degassing pressure and the boiling point. The absolute pressure may also itself be an operating parameter of a device for extracorporeal treatment of blood.
The local ambient pressure may be between 700 hPa and 1060 hPa, for example, depending on the geographic location and the weather situation. An automatic adjustment in the operating parameters depending on the absolute pressure would also be desirable in the case of weather-related changes in the absolute pressure.
The degassing pressure must be set for a safe and reliable degassing of the clean water. The dissolving behavior of gases in liquids depends on the absolute pressure. The saturation concentration decreases with a decline in the absolute pressure. In generic blood treatment devices an absolute degassing pressure of approximately 150 hPa based on the vacuum is the goal. The degassing pressure is generated by a degassing pump and a throttle in the clean water line of the dialysis fluid system. Without an absolute pressure measurement, the degassing pressure can only be determined approximately. For a safe and reliable degassing, the most accurate possible setting of the degassing pressure is desirable.
The boiling point is needed to prevent steam from forming in hot cleaning of the dialysis fluid system because when steam forms it changes the flow properties in the dialysis fluid system and the steam can escape from the dialysis fluid system and enter the interior of the housing of the blood treatment device, where the steam can condense and lead to damage.
The boiling point drops with a drop in pressure. The change in the boiling point with the absolute pressure is known as a qualitative measure and can be obtained from the professional literature, for example, from tables for water and stored in the central control unit of the blood treatment device.
In heat disinfection of the dialysate system, the entire dialysis fluid system must be heated to more than 80° C. in a known device. The entire dialysis fluid system is especially advantageously purified by rinsing with hot water at approximately 85° C. In hot purification, the entire dialysis fluid system is rinsed with heated clean water at a predetermined flow rate. The clean water is heated with an electric heating rod. To keep the heating time of the clean water as short as possible, the clean water temperature on the heating rod should be as high as possible. The clean water temperature on the heating rod in the dialysis fluid system is always regulated at a temperature below the boiling point.
The absolute pressure may be appropriate for determining additional operating parameters. For example, in another patent application by the present applicant with the title “Method and Device for Testing the Delivery Power of at Least One Delivery Agent of a Device for Extracorporeal Blood Treatment”: (internal application Ser. No. 10/57-d01 DE) with the same filing date as the present application, a method and a device for which knowledge of the absolute pressure is advantageous are described. Reference is thus herewith made to the full extent to the aforementioned other patent application.
For determining and providing many operating parameters, however, the existence of a current measured value of the absolute ambient pressure, also referred to simply as absolute pressure, would be advantageous. If there is a measured value for the absolute pressure, such operating parameters can be determined and adjusted with a particularly high precision.